Integrated circuits have become increasingly important in recent years through increased usage and expanded product applications. Chemical compound semiconductor devices, such as gallium arsenide (GaAs) integrated circuits are widely used in microwave circuits, communication satellite systems, and information network systems. This popularity is due, in part, to the fact that GaAs integrated circuits exhibit outstanding high-speed properties and high-frequency characteristics. Multiple microelectronic devices, usually including gate electrodes and ohmic electrodes, are created on the surface of a single wafer or substrate by the successive application of various fabrication techniques. When a device utilizes a GaAs crystal substrate, usually gold or gold alloys are used in metallization, and the device is patterned using a lift-off technique. This is generally a three-part process during which a photoresist is produced, one or more layers of film or metallization are deposited, and finally the resist is stripped off by a wet chemical stripper, lifting off the unrequired portions of the metallization.
During fabrication, the resist mask produces a steep profile. As a result, the film or metallization deposited may overhang the underlying resist, but ideally, the deposited film does not completely cover the edges of the resist steps. This failure of step coverage in the overhanging profile provides an opportunity for the wet chemical stripper, such as an organic solvent, to invade the gaps in the metallization and the resist to dissolve the resist and lift away the overlying metal plating. If the deposited plating completely covers the end of the resist, or the photoresist coverage at the main surface of the substrate is incomplete, there may be inadequate opportunity for the chemical stripper to dissolve the resist. As a result, portions of the plating may remain on the surface of the substrate. If the photoresist pattern is not completely removed at this time, it may contaminate diffusion or deposition furnaces. Additionally, these partially detached metallic remains may cause variations in the thickness of the photoresist film in subsequent photoresist plating processes. Further, inconsistent photoresist lift-off often produces defective circuits, thereby reducing yield.